THE MECHANICS OF CLASTIC INTRUSION: IMPLICATIONS FOR DEEP WATER CLASTIC RESERVOIRS1 Lidia Lonergan, and Richard J.H, Jolly* T.H. Huxley School, Imperial College, London, UK *Now at Golder Associates UK Ltd, Maidenhead, England [email protected] intrusions. However, our work suggests that large-scale Abstract (several hundred metre) sandstone intrusions within Several productive Paleogene deep water sand- Paleogene deep marine successions of the North Sea, stone reservoirs in the North Sea (e.g. Alba, Forth/ require the presence of fluids migrating from deeper Harding, Balder, Gryphon) show evidence of having within the basin (e.g. gas charge) to drive the injection. undergone major post-depositional remobilisation and clastic injection, which can result in disruption of pri- mary reservoir distribution. Remobilization features, Introduction range from centimetres (e.g. core-scale) to hundreds of metres (e.g. seismic-scale). The scale of the clastic in- Cubic-kilometre sized clastic intrusions and remo- trusion and remobilisation has significant impact on res- bilized sandstones are increasingly recognised as an ervoir architecture and production performance, includ- important component of the deepwater play within the ing changes in (a) original depositional geometries; (b) latest Paleocene and early Eocene of the North Sea ba- reservoir properties; (c) connectivity, (d) top reservoir sin. Sediment deformation and intrusion are not just lo- surface structure, (e) reservoir volumetrics, and (f) re- calized processes which have affected one or two fields covery/performance predictions. in the North Sea, but within the late Paleogene interval, There are two prerequisites for sandstone in- remobilization and intrusion of sands has directly af- trusions to form: the source sediment must be fected the identification, definition, and understanding uncemented, and the ‘parent’ sand body must be sealed of at least 10 important hydrocarbon reservoirs and pros- 2 such that an overpressure with a steep hydraulic gradi- pects within an area of ~500,000 km in the Central and ent can be generated. The seal on an overpressured sand Northern North Sea (e.g. Forth/Harding, Alba, Balder, must then be breached for the sand to fluidize and in- Gryphon fields; Alexander et al. 1993; Jenssen et al., ject. The stress state within the basin, burial depth, fluid 1993; Newman et al. 1993; Newton & Flanagan, 1993; pressure and the nature of the sedimentary host rock all Timbrell, 1993; Dixon et al. 1995; Lonergan & contribute to the final style, geometry and scale of in- Cartwright, 1999; Lawrence et al. 1999; MacLeod et al. trusion. At shallow depths, within a few meters of the 1999). Figure 1 illustrates an example of a kilometer – surface, small irregular intrusions are generated, more scale intrusion, consisting of a dike and sill imaged on commonly forming sills, whereas at depth dikes and sills 3D seismic data within Eocene strata, from the North- form clastic intrusion networks. We use field examples ern North Sea. The intrusion cross-cuts 200 m of stratig- from the western Ireland, and Santa Cruz and Panoche raphy, and the presence of sand in this intrusion has been Hills in California to illustrate the control of burial depth/ verified by drilling. (More examples of both core, and stress on intrusion scale. The cohesivity of the host sedi- seismic-scale intrusions from the North Sea are described ment, and the flow velocity of the intruding sands ap- in Lonergan et al. submitted). pears to control whether the intrusion is emplaced by Traditionally, the approach adopted when interpret- stoping (the incorporation of host rock material as rafts ing Tertiary deep-water sandstone reservoirs in the North in the intrusion), or dilation (the forceful pushing apart Sea, has been to assume that the current reservoir ge- of the host rock to create space), resulting in diverse ometry and heterogeneity is largely a function of an origi- styles of intrusive geometry. nal primary depositional origin. We argue, that there is Earthquake induced liquefaction, tectonics and now a mounting body of evidence which suggests that build up of excess in situ pore pressure are the most com- this paradigm be reassessed and that in other deepwater monly cited explanations for the occurrence of clastic settings, the possibility that observed sandstone geom- etries might be a function of post-depositional 1This contribution includes material from two papers currently in review: Jolly, R.J.H. & Lonergan, L. The mechanisms of clastic sill and dike intrusion,, GSA Bull. and Lonergan, L. et al. Remoblisation and Injection in Deepwater Depositional Systems- Implications for reservoir architecture and prediction, GSSEPM- Deep Water Reservoirs of the World Conference Publication Stanford Rock Fracture Project Vol. 11, 2000 C-1 Dyke Balder Marker Sill SAND 100 m ~1 km Figure 1. Seismic section illustrating a kilometer-scale dike and sill complex in the North Sea, within Eocene strata. remobilisation, needs to be evaluated. To further 2a): Remobilisation leads to fundamental understand and predict geometries of intrusions changes in reservoir architecture e.g. that form in deepwater sedimentary environments steepening of original depositional geom- we need an improved theoretical basis from which etries (e.g. Balder Field, Jenssen et al. to consider the process of clastic intrusion and 1993; Rye-Larsen, 1994), development of the resultant geometries. The mechanical control pod-like sandbodies (e.g Balder and Alba on intrusion geometry, size and intrusion mecha- Fields), intrusion of clastic dikes and sills nism is the main topic of this paper. above the reservoir (e.g. Forth/Harding, Frigg, Gryphon and Alba Fields; Dixon et al., 1995; Newman et al. 1993; Newton & Remobilisation and the effects on res- Flanagan, 1993) and laterally along the reservoir margins (e.g. Alba Field, ervoir geology Lonergan & Cartwright, 1999; MacLeod As a direct result of remobilisation and injec- et al. 1999). tion, many of the North Sea Paleogene reservoirs (b) Changes in reservoir properties (Figure have complex geometries. This, coupled with their 2b): Remobilization often homogenizes subtle expression on seismic data, and with their reservoir properties (e.g. by clay lack of primary depositional characteristics com- elutriation) and eliminates original sedi- bine to make them challenging prospects for ex- mentary structures leading to a massive ploration and appraisal. Sand remobilisation pro- sandstone facies. These facies are often cesses can affect reservoir geology in a number considered an original depositional facies of ways: and then interpreted as the deposits of ei- (a) Changes in reservoir architecture (Figure ther sandy grainflows or debris flows. C-2 Stanford Rock Fracture Project Vol. 11, 2000 A. Change in reservoir geometry B. Change in reservoir properties Massive blocky sandstone intra-reservoir shales Channelised No internal turbiditic sandstones clay breaks C. Change in connectivity Dike OWC Sill Isolated channel sands D. Change in top reservoir surface & in reservoir volumetrics Producer No Yes OWC OWC Figure 2. Schematic diagram illustrating the potential effects of clastic intrusion and remobilization on reservoir geol- ogy. (c) Change in connectivity of originally iso- bedding (sills) (Figure 3). Jolly and Lonergan (sub- lated reservoirs (Figure 2c): Clastic intru- mitted) review the extensive published literature sions alter the transmissivity of the reser- spanning a century and more, on the occurrence voir and connectivity between previously of clastic dikes and sills. While clastic intrusions isolated reservoir units can be established. have been documented from all depositional en- Vertical or steeply dipping dikes will not vironments, they have been most frequently re- be imaged on seismic data, so connectiv- corded in deepwater depositional settings. Clas- ity between apparently separate sand bod- tic intrusions documented in outcrop are typically ies may not be evident. small, rarely reaching tens of metres thickness in (d) Changes in top reservoir surface and in dimension, but have never been observed at the reservoir volumetrics (Fig 2d) (e.g. Alba scale of those interpreted from seismic data in the Field, MacLeod et al., 1999). North Sea. The largest known outcrop examples occur within tectonically active basins, such as the large oil-bearing intrusions exposed near Santa Clastic injection and intrusion trigger- Cruz in California within the Miocene Santa Cruz mudstone (see Thompson et al. 1999 for a recent ing mechanisms description) in the Santa Cruz/La Honda strike slip The emplacement of remobilised clastic sedi- basin along the San Andreas fault, or from thrust ment into the surrounding strata, can either form belt/accretionary prism settings (e.g. Winslow tabular bodies of sediment that are discordant to 1983; Scott, 1966) bedding (dikes), or sheets largely concordant with The majority of previously published work Stanford Rock Fracture Project Vol. 11, 2000 C-3 m. (see references cited in Obermeier 1996). Thus, ? ? when considering earthquake liquefaction as a sill length potential trigger for clastic intrusion it is impor- T tant to consider the scale of the intrusion, the depth Step at which the intrusions formed, and the likelihood dyke that earthquakes greater than magnitude 5 may height Jog w L have occurred at the time of intrusion (i.e. was the basin in an active plate tectonic setting?). Source bed Fluid- induced liquefaction, where the fluid is not the in-situ pore fluid, but migrates into the sealed sand body from elsewhere in the basin, has rarely been cited as a mechanism for triggering Figure 3 Diagram illustrating principle features of clas- clastic intrusions. Jenkins (1930) recognised that tic sills and dikes intruded in the subsurface. W= dike the migration of hydrocarbon fluids potentially width, L= dike length (strike length if exposed in out- played an important role in the formation of the crop), T= sill thickness. Note the blunt termination at large number of dikes found in the oil-producting one end of the illustrated sill and the thin fingers (2D); basin of California. Thompson et al. (1999) sug- sheets (3D) at the other end.